1. Field of the Invention
The invention generally relates to optical disc recording or reading systems and, more particularly, to a method of and a system for modulating a series of 8-bit data into a 15-bit code stream that includes no merging bit for digital data recording on an optical disc and to a demodulation method and system adapted for the modulation method and system.
2. Description of the Prior Art
Generally speaking, in digital data recording or transmission, each byte of recorded digital data is modulated into a code such that the characteristics of an actually recorded code sequence (or recorded signal) matches those of the recording medium or the transmission channel, respectively, which is well known as the "channel coding". In case of magneto-optic recording, the modulation or channel coding is such that the recorded signal satisfies the following constraints: a minimum zero-run-length constraint (i.e., a minimum pit or land length constraint) due to optical transmission characteristics in recording and reading and to physical restrictions involved in pit formation, a maximum zero-run-length constraint (i.e., a maximum pit or land length constraint) for the facilitation of timing recovery, and a constraint on low-frequency components for the sake of servo control. The first two constraints are expressed en bloc as the (d,k) or run-length-limited (RLL) constraints, where the run of "0" symbols between consecutive "1" symbols in a recorded signal has length at least d and no more than k. The last constraint is judged by the digital sum value (DSV) as well known in the art.
Widely used RLL constraints are the (2,10) constraints, where 0-runs in the code have length at least 2 zeros and no more than 10 zeros, i.e., 0-runs have length at least 3T (T is a channel bit period) and no more than 11T. Actually used modulation schemes according to constraints (d,k)=(2,10) are, for example, eight-to-fifteen modulation (EFM) for the Compact Disc (CD) and EFMPlus (eight-to-sixteen modulation) for the Digital Versatile Disc (DVD).
Variety of other modulation schemes according to RLL constraints have been proposed so far. Some of them are as follows.
(1) Brian H. Marcus, Paul H. Siegel and Jack K. Wolf, "Finite-State Modulation Codes for Data Storage," IEEE Journal on selected areas in communications, Vol. 10, No. 1, January. 1992, pp. provides a self-contained exposition of modulation code design methods based upon the "state splitting algorithm."
(2) Japanese unexamined patent publication No. 08031100 (1996) discloses "Method and Device for Modulating Signal and Signal Demodulation Device," in which 8-bit source words are modulated into a 16-bit code stream that includes no merging bit.
(3) Japanese unexamined patent publication No.08077717 (1996) discloses "Digital Modulation and Demodulation Method, Device therefor, Recording Medium, and its Production," in which 8-bit source words are modulated into a series of 15-bit blocks each of which comprises a 14-bit code and one "merging bit." In this modulation system, the code bit three bits before the merging bit and/or the code bit three bits after the merging bit is (or are) so adjusted as to satisfy the minimum run length constraint and minimize the DSV.
(4) Japanese unexamined patent publication No.08149013 (1996) discloses "Digital Modulating and Demodulating Method and Device for the Method," in which 8-bit source words are modulated into a series of 15-bit blocks each of which comprises a 14-bit code and one "merging bit". In this modulation system, one of the surplus codes (the codes not used) in EFM is used in place of a selected code, if possible, such that the DSV becomes minimum.
All of the above-cited documents are hereby incorporated by reference.
However, there still remains a room for improvement in the art. It is therefore an object of the invention to provide an eight-to-fifteen modulation code system (or table), and an eight-to-fifteen modulation method and system that uses the modulation code table and that provides a 15-bit code stream satisfying the (2,10) RLL constraints and the DSV constraint and including no merging bit. SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a code table for use in converting each source code of a p-bit source code sequence into a q-bit (p&lt;q) object code whose NRZI-converted version satisfies predetermined run-length-limited (RLL) constraints in a p-to-q modulator which simply concatenates the converted q-bit object codes into a q-bit object code sequence that still satisfies the run-length limited constraints. The code table comprises a column for storing 2.sup.p possible p-bit source codes in order of magnitude and a plurality of columns (hereinafter referred to as "subtables"). Each subtable comprises a subtable ID for identifying the subtable and 2.sup.p combinations of 2.sup.p q-bit object codes and respective next subtable fields which combinations are associated in the row directions with respective source codes. Each of the next subtable fields contains a subtable ID of a subtable to be used in a modulation of a next source of the source code sequence so as to cause the source code sequence to satisfy the RLL constraints. Object codes are permitted to appear repeatedly in the 2.sup.p q-bit object codes in each subtable such that combinations of appearances of each of the overlapping object codes and respective next subtable fields are unique in the subtable. The subtables include at least one group of a first subtable and at least one second subtable such that the first subtable can be used for at least one second subtable for source codes of the 2.sup.p source codes that are in a predetermined range and that, for each of the source codes in the predetermined range, the number of "1" symbols of two corresponding object codes of the first subtable and each of the second subtable(s) is an odd number.
In one embodiment, p and q are 8 and 15, respectively the RLL constraints are such that the run lengths of an NRZI converted signal of an object code sequence are limited to 3T through 11T, where T is a channel bit period of each object code. Also, the source codes in the predetermined range are set to 0 through 38.
According to another aspect of the invention, a method of and a system for modulating a p-bit source code sequence into a q-bit (p&lt;q) object code sequence that includes no merging bit and whose NRZI converted version satisfies predetermined run-length-limited constraints. It is assumed that the method and the system use the above-described code table. A first object code associated with a source code of the source code sequence is retrieved from a subtable identified by a next subtable field attached to the last modulated object code. A test is made to see if the subtable identified by the next subtable field is one of the second subtable(s) and the source code is in the predetermined range. In response to a pass in the test, a second object code associated with the source code of the source code sequence is retrieved from the first subtable in the same group as that of the second subtable. A first digital sum value (DSV) and a second DSV are calculated by using the first source code and the second source code, respectively. If the absolute value of the second DSV is smaller than that of the first DSV, then the second object code is included in the q-bit object code sequence. And if the test is unsuccessful or if the absolute of the second DSV is not smaller than that of the first DSV, then the first object code is included in the q-bit object code sequence.
According to further aspect of the invention, a method of and a system for demodulating a q-bit code sequence into a p-bit (p&lt;q) code sequence is provided. It is assumed that the q-bit code sequence is obtained by reading an optical disc on which information is recorded according to the above-mentioned modulating method or system. In addition to the above-described code table, the method and the system use state calculating means, based on a value of a next q-bit code following a current q-bit code, for calculating at least one subtable from which the next q-bit code might be derived to provide a calculated subtable ID's and a possible state table for associating each of ending types determined by respective ranges of the 0-run length on the LSB (least significant bit) side (hereinafter, referred to as "LSB 0-run length") of the current q-bit codes with ID's of possible subtables permitted to the ending type. ID's of the permitted subtables are found based on the LSB 0-run length of the current q-bit code. The calculated subtable ID's are obtained from the abovementioned state calculating means. An ID common to the ID's of the permitted subtables and the calculated subtable ID's is found as a next subtable ID for the next q-bit code. If a plurality of such common ID's are found, any of the found ID's may be used. A subtable identified by a next subtable ID for the current q-bit code is searched for a combination of the current q-bit code and the next subtable ID for the next q-bit code. A p-bit code in a row in which the combination is located is output as a demodulated code.
According to still another aspect of the invention, a method of and a system for demodulating a q-bit code sequence into a p-bit (p&lt;q) code sequence is provided. It is assumed that the q-bit code sequence is obtained by reading an optical disc on which information is recorded according to the above-mentioned modulating method or system. The method and the system use state calculating means, based on a value of a next q-bit code following a current q-bit code, for calculating at least one subtable from which the next q-bit code might be derived to provide a calculated subtable ID's and a decoding table for associating each of possible q-bit current codes with an ending type and corresponding p-bit codes further associated with respective subtable ID's for use in a demodulation of the next q-bit code. ID's of the permitted subtables are found based on the ending type associated with the q-bit current codes. The calculated subtable ID's are found from the above-mentioned state calculating means.
An ID common to the ID's of the permitted subtables and the calculated subtable ID's is found as a next subtable ID for the next q-bit code. If a plurality of such common ID's are found, any of the found ID's may be used. A p-bit code associated with the current q-bit code and the next subtable ID for the next q-bit code is obtained from the decoding table and output as a demodulated p-bit code.
In one embodiment, p and q are 8 and 15, respectively the RLL constraints are such that the run lengths of an NRZI converted signal of an object code sequence are limited to 3T through 11T, where T is a channel bit period of each object code.